This invention relates to communication systems. Specifically, the invention is directed to the processing of a directed retry instruction received by a mobile station operating within a cellular network.
A description of cellular radio systems is found in Cellular Radio Systems, published by Artech House, Boston (editors: D. M. Balston and R. C. V. Macario; ISBN: 0-89006-646-9); Digital Cellular Radio written by G. Calhoun and published by Artech House, Boston (ISBN: 0-89006-266-8); and standard TIA/EIA-136 Rev A 800/1900 MHz TDMA Cellular-Radio interface-Mobile Station-Base Station Compatibility, Nov. 20, 1998. Cellular Radio Systems, Digital Cellular Radio, and xe2x80x9cTIA/EIA-136 Rev Axe2x80x9d are hereby incorporated by reference in their entirety.
A cellular mobile telecommunications system includes a mobile station communicating with any one of a plurality of geographically spaced base stations. Broadly, each base station defines a cell, and each cell forms an integral part of a larger cellular network. The size of a cell largely depends on the power rating of the corresponding base station. The base stations communicate with a mobile switching station by means of intercellular trunk lines. The mobile switching center determines which of the base stations and channels should process a call with the mobile station based on considerations such as signal strength between each available channel and the mobile station.
When the mobile station has a digital control channel (xe2x80x9cDCCHxe2x80x9d) or an analog control channel (xe2x80x9cACCxe2x80x9d), the mobile station can receive or initiate a call. The terms xe2x80x9cinitiatexe2x80x9d and xe2x80x9coriginatexe2x80x9d will hereafter be regarded as equivalent terms. Specifically, the mobile station can receive or originate a call via a channel when the signal strength is satisfactory between the mobile station and a base station. If the mobile station travels out of a first cell into a second cell and there is no channel available in the second cell, the mobile station can attempt to keep the channel provided by the base station in the first cell. Sometimes the geography between first cell and the second cell is such that the signal strength is compromised. In these circumstances the mobile station can attempt to find an unused channel on a base station in a third cell. Switching between base stations can be handled by a switching station in communication with a service provider.
The mobile station can maintain information concerning base stations (xe2x80x9cneighbor stationsxe2x80x9d) in neighbor cells in the form of a neighbor list. The mobile station can verify signal strength, i.e. make a xe2x80x9cneighbor list measurementxe2x80x9d, between itself and a neighbor station. The industry requirement is to take at least five valid neighbor list measurements at intervals prior to making a decision to switch to a new control channel.
If another caller is making a page attempt to the mobile station or the mobile station is attempting a call and the services within a cell are saturated, the call may fail. To avoid this, the cellular network can broadcast via a base station a directed retry message to the mobile station. This will cause the mobile station to tune to a different channel. On receiving the directed retry message (see e.g. TIA/EIA-136-140), the mobile station can select a neighbor station from the neighbor list and thereby invoke a reselection trigger condition type 4 (xe2x80x9cRTC-4xe2x80x9d) to initiate a Candidate Eligibility Filtering procedure in order to attempt a channel reselection.
After the mobile station invokes the RTC-4 condition, it can initiate the process of responding to a page or completing a call attempt as is well known in the art. Unfortunately, timeout can occur if the mobile station receives a directed retry message while in the process of obtaining a new set of neighbor list measurements. Instead of responding to a directed retry message by invoking RTC-4 condition, the call origination attempt or page attempt fails and the mobile station reverts to camping or idle mode. Alternatively, the neighbor list might be stale and the mobile station is about to update the neighbor list when the mobile station receives a directed retry message causing the would be incoming page or outgoing call to fail causing the mobile station to revert to camping mode without directed retry.
In normal circumstances there is usually enough capacity in the cell on which the mobile station is camped, a directed retry message is not broadcast and the incoming page or outgoing originated call is processed by the mobile station. Specifically, the page attempt or call is successful and the mobile station uses the cell base station to handle the page or call communication.
The present invention is directed toward to overcoming one or more of the problems discussed above in a novel and simple manner.
In accordance with the invention there is provided a system and method of handling a directed retry message to significantly increase the chances of establishing a channel of communication between the mobile station and base station.
Broadly, there is disclosed herein a method of rapidly processing a directed retry message received by a mobile station while the mobile station does not have an up-to-date neighbor list or is in the process of completing a set of neighbor list measurements. The method comprises the steps of receiving a directed retry message on a mobile station; if an up-to-date neighbor list does not exist on the mobile station, setting SCANINTERVAL to zero and HL_FREQ to high; completing at least one neighbor list measurement; and invoking RTC-4. The step of completing the least one neighbor list measurement is accomplished within a wait time of about four seconds. The step of completing the least one neighbor list measurement preferably comprises completing two-neighbor list measurements. If the wait time exceeds about four seconds the mobile station reverts to camping or idle condition.
It is a feature of the invention to process a directed retry message immediately after channel selection.
It is a feature of the invention to reduce the time taken to calculate neighbor measurements on a mobile station that has received a directed retry message immediately after selection.
It is a feature of the invention to provide an improved method for increasing the chance of a successful page or call origination when a mobile station receives a directed retry message.
It is a feature of the invention that a mobile station can invoke RTC-4 condition with fewer than five neighbor list measurements.